Transform a Keystone token to a SAML assertion

Add a class to generate a SAML assertion, based on Keystone
attributes.

Co-Authored-By: Marek Denis <marek.denis@cern.ch>

Implements bp keystone-to-keystone-federation
Change-Id: Iff82b4310d10b70fa6447ec52a504ef1f876cac4
Merge "Transform a Keystone token to a SAML assertion"